1. Field of the Invention
This invention relates to gas turbine engines and more particularly to the rotor assemblies of such engines.
2. Description of the Prior Art
A gas turbine engine has a fan section, a compressor section, a combustion section and a turbine section. A rotor assembly having a compressor segment and a turbine segment extends axially through the engine. A shaft connects the turbine segment with the compressor segment. In the compressor segment rows of rotor blades extend outwardly on the assembly. A stator assembly circumscribes the rotor assembly. The stator assembly includes an outer case and rows of stator vanes extending inwardly between axially adjacent rows of rotor blades. Struts extend inwardly from the outer case to support bearing compartments at two or more locations. Each bearing compartment houses a bearing at the axis of the engine for support of the rotor assembly.
The compressor segment has a plurality of axially spaced disks. Each pair of disks is joined in spaced relationship by a spacer disposed therebetween. The spacers and the portions of each disk between the spacers form the rotor backbone of the engine. In an alternate rotor construction a wing member integral with one disk functionally replaces the adjacent spacer in forming the rotor backbone. The rotor backbone is designed to be concentric with the axis of the engine.
Rotor loads and gyroscopic maneuver loads during operation are known to cause the rotor to deflect from the intended concentric position. The deflection is directly proportional to the weight of the rotor assembly and the distance between bearing supports. Some engine designers have attempted to decrease deflections by shortening the distance between bearing supports. For example, in structures of the type respresented by U.S. Pat. No. 2,869,820 to Marchant et al. entitled "Rotors for Axial Flow Compressors or Turbines", the unsupported length of the compressor segment is reduced by housing one of the bearings within the blade-carrying length of a rotor. Others have attempted to reduce rotor weight.
As the rotor deflects, the axis of rotation and the center of mass of the rotor no longer coincide and an eccentricity of mass E about the centerline occurs. The eccentricity of mass E results in an outwardly directed force which increases as the speed of rotation of the rotor assembly increases. The outwardly directed force tends to bend the rotor assembly. At a certain speed, commonly called the critical speed, the unbalanced outwardly directed force neutralizes the elastic forces of the rotor which resist deflection. At the critical speed, rotor deflection increases with no further increase in speed. The tips of the rotor blades rub against the outer case causing considerable damage. Accordingly, the rotors of the gas turbine engines are typically designed to have critical speed characteristics well above the operational speeds of the rotors.
Scientists and engineers are continuing to search for structures for lightweight rotor assemblies having a critical speed characteristic well above the operational speed of the rotor assemblies.